The Property List (plist) file is a standard feature of the operating systems descended from NeXTSTEP, which includes the current Apple operating systems. It is the standard way for programs developed under such systems to save and load data between the internal representation within the objects defined in an Objective-C program and disk files, in order to store program settings and other data. The Cocoa framework's standard objects have built-in methods to deal with these files. NeXTSTEP had a text-based format for this, in a style resembling a programming language; Apple replaced it with an XML-based format for Mac OS X, and later on they also introduced a binary format as well, which is now the default save format for preference files.
Current MacOs development frameworks can also use property lists in JSON, but these can't necessarily be precisely transferred to and from the other formats, since the data types aren't exactly the same.